1. Field of the Invention
The present invention relates, in general, to the demodulation of constant-amplitude biorthogonal modulation signals and, more particularly, to a demodulation method, which enhances reception performance using the characteristics of the modulation signals.
2. Description of the Related Art
Recently, Spread Spectrum (SS) systems have been used as the physical layers of important wireless Local Area Network/Personal Area Network (LAN/PAN) due to the characteristic thereof of being resistant to interference. For example, Institute of Electrical and Electronics Engineers (IEEE) 802.11 and IEEE 802.11b, which are wireless LAN standards, adopt a Direct Spreading (DS) scheme and a Complement Code Keying (CCK) scheme, respectively. Further, IEEE 802.15.4, which is a wireless PAN standard, uses orthogonal modulation, and Ultra-WideBand (UWB) uses biorthogonal modulation.
In a Code Division Multiple Access (CDMA) scheme used in mobile communication, a Direct Sequence (DS)/CDMA scheme of directly multiplying data by orthogonal codes to achieve a spread spectrum is mainly used. However, such a spread spectrum system is disadvantageous in that a spectrum is wasted due to spreading, so that the system is not suitable for high speed data transmission. Therefore, research on a spread spectrum system for providing high speed transmission has been actively carried out. In the outcome of the research, the most universal scheme for providing high speed transmission is to use a multi-code signal. Multi-code modulation is advantageous in that it can obtain higher spectral efficiency compared to a conventional spread spectrum system, but is disadvantageous in that it requires an expensive power amplifier that provides a wide linearity region so as to amplify a multi-level signal. If the linearity region of the power amplifier cannot cover the output levels of a multi-level signal, the power amplifier may undesirably influence the performance of an entire multi-code system due to the non-linearity of the amplifier. Therefore, in order to use a power amplifier having a narrow linearity region, it is preferable that a multi-code signal have constant-amplitude characteristics.
For schemes of solving various problems occurring when the signal level of a modulation signal increases due to the use of a multi-code signal with the increase of the number of transmission data channels in this way, a Pulse Width (PW)/CDMA scheme (disclosed in Korean Pat. Registration No. 293128), a Multi-Phase (MP)/CDMA scheme (disclosed in Korean Pat. Application No. 10-2001-8033), and a Constant-Amplitude multi-Code Biorthogonal Modulation (hereinafter refer to as “CACB”) scheme designated as a “Code Selection (CS)/CDMA” (disclosed in Korean Pat. Application No. 10-2001-0061738 and Korean Pat. Application No. 10-2002-0020158) have been proposed.
The PW/CDMA scheme is a method of clipping the levels of an output symbol of a digital adder above a certain value (level limitation), converting only the remaining levels into a pulse width, transmitting the pulse width and causing a signal waveform to consistently have a binary form. An output symbol is converted into a pulse signal having a width determined according to a level by a pulse generator. The PW/CDMA is advantageous in that a modulation signal is converted into a binary form, but is disadvantageous in that, if the number of clipped levels of the modulation signal exceeds 2, the bandwidth of the modulation signal increases in proportion to the number of levels.
The MP/CDMA scheme uses M-ary Phase Shift Keying (MPSK) modulation so as to allow a multi-level signal to be transmitted with a constant amplitude. At this time, the number of levels of the signal is limited to a certain number prior to modulation so as to minimize the influence of channel noise. However, when the level limitation is performed in this way, the orthogonality of a signal is damaged to deteriorate performance. Therefore, this method is problematic in that a code selection algorithm definitely influences a Bit Error Rate (BER), and satisfactory BER performance cannot be obtained in the case where the number of codes to be used increases (that is, in the case where considerably high spectrum efficiency is implemented) due to the interference between multiple codes and loss caused by clipping.
The CACB scheme of the above schemes modulates data by selecting one of the orthogonal codes allocated to blocks using data to be transmitted. Since the number of codes to be stored greatly increases if the number of channels increases, a CACB system is implemented by dividing codes into a plurality of blocks. At this time, since orthogonal codes output from respective blocks are added, a modulation signal also becomes a multi-level signal. The CACB system represents a system, which causes the level of an output symbol to be constant by suitably encoding an input information bit stream so as to solve the problem, so that a level limiter is not necessary.
The above CACB technology is described in brief with reference to the attached drawings.
FIG. 1 illustrates the construction of a transmitter of a CACB system having a constant-amplitude encoder. An encoding method performed by this system includes the steps of converting a serial input information bit stream (user data) composed of N bits into N parallel bits using a Serial/Parallel (S/P) converter 110 and grouping the N parallel bits into three blocks to allow k+1 information channels (FIG. 1 illustrates an example in which k is 2 selected from natural numbers, but the present invention is not limited to this example) to be input to each of three biorthogonal modulation blocks 130_I, 130_J and 130_K, and encoding the information bit stream, input to the three blocks, using a constant-amplitude encoder 120 to generate k+1 encoding output bitstream that are to be input to a fourth block 130_L. As described above, the modulator of the CACB system equipped with the constant-amplitude encoder includes four blocks having the same structure, the four blocks each using a Walsh-Hadamard code as an orthogonal code.
Each of the blocks has k+1 input channels to which information bits having data expressed by 0 and 1 are input. Each of the orthogonal modulators 132_I, 132_J, 132_K and 132_L of the blocks selects one of 2k orthogonal codes on the basis of k pieces of input channel information (that is, k bit data). Each of these orthogonal codes has a length of 2k+2 chip, and elements of 1 or −1. Each of the multipliers 134_I, 134_J, 134_K and 134_L of the blocks converts “0” of an information bit input through the remaining one channel into “−1” to generate a bipolar signal, multiplies the orthogonal code, selected by a corresponding orthogonal modulator 132_I, 132_J, 132_K and 132_L, by the bipolar signal, and applies the multiplication results to a digital adder 140.
A method of selecting four orthogonal codes from the four blocks may be described by selecting four rows from an M×M Hadamard matrix. Since a code is selected using k bits per block, 2k codes exist in each block. Since a total of 4 blocks exist, the size M of the Hadamard matrix is M=2k+2. For example, when a code is selected using two bits in each of the blocks (that is, k=2), the size of the Hadamard matrix is 16×16, and a selected orthogonal code has a length of 16chips.
A method of encoding a bit stream input to three other blocks to generate bits that are to be input to the redundant block 130_L in a CACB system having a constant-amplitude encoder is described. In this case, the number of data bits input to each block is assumed to be k+1. One of 2k orthogonal codes is selected using k bit data of k+1 bits, and the selected orthogonal code is multiplied by the remaining one bit data. If 3×(k+1) information bits input to the three biorthogonal modulation blocks 130_I, 130_J and 130_K are encoded and then input to the redundant block 130_L, the amplitude of an output symbol Sq can be maintained at a constant level when the bits output from the four blocks are added to each other by the digital adder 140.
Referring to FIG. 1, among an input information bit stream composed of 9 bits (N=9), bits used to select codes are (b1, b2), (b4, b5), and (b7, b8), and bits used to determine a sign are b0, b3 and b6. A bits stream used to select a code in the constant-amplitude encoder block 130_L, which is the redundant block, is (p1, p2), and a bit used to determine a sign is p0. If the code selection bits p1 and p2 of the redundant block 130_L and one bit p0 used to determine the sign thereof are encoded as expressed in the following Equation [1], a transmission signal becomes +2 or −2, thus causing the amplitude of the output symbol to be constant.p0= b0⊕b3⊕b6, p1=b1⊕b4⊕b7, p2=b2⊕b5⊕b8  [1]
In brief, the CACB system having a constant-amplitude encoder uses Walsh-Hadamard orthogonal codes and includes four blocks. Information bits are transmitted to three blocks among the four blocks, and parity bits formed by encoding the information bits, input to the three blocks, are provided to the remaining one block.
However, a conventional reception or demodulation apparatus demodulates information bits through three blocks except for the redundant block having parity bits. That is, in the conventional reception or demodulation apparatus, the redundant data of one block that are obtained by encoding information bits so as to cause the level of an output symbol of a CACB system comprised of four blocks to be constant and are additionally transmitted are treated as unnecessary data.